1. Field of the Invention
The present invention relates to a reconfigurable unit for compensating the chromatic dispersion, and optionally, the chromatic dispersion slope, comprising a holey optical fibre and a temperature adjusting device.
Moreover, the present invention relates to an optical communication line and an optical communication system comprising such unit, and a method for compensating chromatic dispersion, and optionally, the chromatic dispersion slope, in a reconfigurable manner.
In the present description and claims, the expression:
xe2x80x9cchromatic dispersion coefficient Dxe2x80x9d is used to indicate the dependence at the first order of group velocity on the wavelength. More in particular, the chromatic dispersion coefficient D is expressed by the following relation (Govind P. Agrawal, xe2x80x9cNonlinear Fiber Opticsxe2x80x94Second Editionxe2x80x9d, Academic Press, pages 8-10)   D  =                    ⅆ                  β          1                            ⅆ        λ              =                  -                              2            ⁢            π            ⁢                          xe2x80x83                        ⁢            c                                λ            2                              ⁢              β        2            
where xcex21 and xcex22 are the propagation constants of the first and second order, respectively, and D is expressed in ps/(nm*Km). Moreover, the chromatic dispersion coefficient D can have a positive or negative value based on the sign of the propagation constant xcex22;
xe2x80x9cchromatic dispersion D*Lxe2x80x9d is used to indicate the chromatic dispersion, expressed in ps/nm, accumulated along an optical transmission fibre having a chromatic dispersion coefficient D, expressed in ps/(nm*Km), and a length L, expressed in Km (such product D*L can have a positive or negative value according to whether the chromatic dispersion coefficient D is positive or negative);
xe2x80x9cchromatic dispersion slope sxe2x80x9d is used to indicate the derivative, with respect to the wavelength, of the chromatic dispersion coefficient D, is expressed in ps/(nm2*Km) and can have a positive or negative value; and
xe2x80x9coptical transmission fibrexe2x80x9d is used to indicate an optical fibre used in an optical communication line or system for transmitting optical signals from a point to the other, located at a considerable distance (for example, at at least some km or tenths of km).
2. Description of the Related Art
In the field of optical telecommunications and optical signal propagation in an optical transmission fibre, chromatic dispersion (or second order dispersion), defined by the above chromatic dispersion coefficient D, is a phenomenon for which different spectral components of a light pulse propagating in an optical fibre travel at different speeds, causing a temporal broadening of the pulse.
In an optical communication system, chromatic dispersion thus limits the maximum data transmission speed (that is, the bit rate) or the maximum connection length without electrical signal regeneration.
In order to compensate chromatic dispersion Dt*Lt accumulated along an optical transmission fibre having a chromatic dispersion coefficient Dt and a length Lt, there are known devices comprising, for example, an optical fibre specifically designed to have a very high value of the chromatic dispersion coefficient Dc, with opposed sign with respect to that of the optical transmission fibre, and such length Lc as to satisfy the relation Dt*Lt=xe2x88x92Dc*Lc.
However, a compensator device of this type, designed so as to compensate a certain value of chromatic dispersion Dt*Lt, is not suitable for compensating the chromatic dispersion of another optical transmission fibre characterised by a different value of product Dt*Lt with respect to that for which the compensator device has been designed.
For example, a compensator device designed to compensate exactly the chromatic dispersion accumulated along a span of 100 Km (Lt=100 Km) of a conventional single mode optical fibre (or SMF) having a value of the chromatic dispersion coefficient Dt equal to about 17 ps/(nm*Km) is not capable of compensating exactly the chromatic dispersion of spans of the same SMF fibre having, however, a length different from 100 Km (for example, 70, 80, 90 or 110 Km).
Moreover, a compensator device designed to compensate exactly the chromatic dispersion accumulated along a span of 100 Km of a conventional SMF fibre is not capable of compensating exactly the chromatic dispersion accumulated along such span of 100 km in case of variations of the chromatic dispersion coefficient Dt of the SMF fibre with respect to the nominal value due, for example, to the variation of system parameters, such as temperature.
Even though in this latter case the variation of the value of chromatic dispersion Dt*Lt of the optical transmission fibre generally is irrelevant in an optical communication system with moderate bit rates, it becomes very important at high bit rates (2,5, 10, 40, 80 Gbit/s), at which an increasingly higher precision of the chromatic dispersion compensation is required.
There is thus the need for a unit for compensating the chromatic dispersion, suitable to be reconfigured so as to compensate, according to requirements, different values of chromatic dispersion Dt*Lt accumulated along an optical transmission fibre.
B. J. Eggleton et al. (xe2x80x9cTunable dispersion compensation in a 160-Gb/s TDM system by a voltage controlled chirped fiber Bragg gratingxe2x80x9d, IEEE Photonics Technology Letters, Vol. 12, No. 8, August 2000, pages 1022-1024) describe an integrated chirp-tunable Bragg grating for compensating chromatic dispersion in a dynamic manner and capable of recovering 2 ps pulses over a 50 ps/nm tuning interval, with a system penalty that is less than 1.3 dB.
S. T. Vohra et al (xe2x80x9cDynamic dispersion compensation using bandwidth tunable fiber Bragg gratingsxe2x80x9d, ECOC 2000) describe a device for compensating chromatic dispersion in a tunable manner, realised with a fibre Bragg grating and capable of compensating chromatic dispersion in a tuning interval from xe2x88x92150 ps/nm to xe2x88x923500 ps/nm.
However, the above devices are not capable of compensating also the third order dispersion or chromatic dispersion slope (or slope) according to which light pulses at different wavelength propagate in an optical fibre with different dispersions.
This phenomenon, caused by the chromatic dispersion being a phenomenon depending on the wavelength, is a problem in wavelength division multiplexing (or WDM) optical communication systems, where information is carried along the same optical fibre by a plurality of optical signals at different wavelength.
Thus, in WDM optical communication systems, it is necessary to compensate not only chromatic dispersion but also chromatic dispersion slope in the interval of wavelengths of interest.
Devices for compensating both chromatic dispersion and chromatic dispersion slope of a conventional single mode fibre (or SMF) are known.
For example, to compensate both a chromatic dispersion coefficient Dt and a chromatic dispersion slope st, there are known devices comprising an optical fibre specifically designed to have very high values of the chromatic dispersion coefficient Dc and of chromatic dispersion slope sc, with opposed sign with respect to those of the SMF optical fibre of which dispersion is to be compensated, so that relation Dt/st=Dc/sc is satisfied (T. Kashiwada et al., xe2x80x9cBroadband dispersion compensating module considering its attenuation spectrum behavior for WDM systemxe2x80x9d, OFC ""99, WM12, pages 229-231; G. E. Berkey et al., xe2x80x9cNegative slope dispersion compensating fibersxe2x80x9d, OFC ""99, WM14, pages 235-237 and L. Gruner-Nielsen et al., xe2x80x9cDesign and manufacture of dispersion compensating fibre for simultaneous compensation of dispersion and dispersion slopexe2x80x9d, OFC ""99, Technical Digest WM13, pages 232-234).
In fact, it is known that both the following relations must be satisfied for compensating both the chromatic dispersion coefficient Dt and the chromatic dispersion slope st
Dt*Lt+Dc*Lc=0
st*Lt+sc*Lc=0
that is, the Dt/st ratio must be equal to the Dc/sc ratio.
Moreover, the dispersive properties of a holey optical fibre have been studied in recent years.
A holey optical fibre typically consists of a single material in which the refractive index difference between core and cladding, which allows guided propagation, is obtained through the presence of holes in the cladding, which lower the refractive index of the material forming the fibre.
More in particular, a holey optical fibre has a cladding region comprising holes that run along the entire fibre length, and a solid core region determined by the absence of at least one hole in the material.
For example, U.S. Pat. No. 5,802,236 patent describes a micro-structured optical fibre which includes a solid silica core region surrounded by a inner cladding region and an outer cladding region. The cladding region has capillary holes that extend in the axial direction of the fibre. The holes in the outer cladding region are of a smaller diameter than the holes in the inner cladding region and therefore the effective refractive index of the outer cladding region is greater than the effective refractive index of the inner cladding region. This document states that this type of fibre may have high negative values of the chromatic dispersion coefficient D (for example, values that are more negative than xe2x88x92300 ps/nm*Km) at a predetermined wavelength (for example, 1550 nm) and high negative values of the chromatic dispersion slope s so that the fibre can carry out a dispersion compensation in a range of wavelengths of 20 nm or more.
However, the above devices suitable for compensating both chromatic dispersion and chromatic dispersion slope of a conventional single mode fibre are not reconfigurable.
The Applicant has posed the technical problem of compensating both chromatic dispersion and chromatic dispersion slope in a reconfigurable manner.
More in general, the Applicant has posed the technical problem of compensating the chromatic dispersion in a reconfigurable manner.
The Applicant has found that the above technical problems can be solved by a unit comprising a holey optical fibre and a device for adjusting the temperature thereof in a predetermined interval of temperatures.
In fact, observing that the chromatic dispersion coefficient Dc and the chromatic dispersion slope sc of a holey optical fibre vary as the temperature varies, the Applicant has found that a holey optical fibre having length Lc can be used to obtain, as the temperature varies, desired values of chromatic dispersion Dc(T)*Lc and of the Dc(T)/sc(T) ratio.
In a first aspect thereof, therefore, the present invention relates to a unit comprising a portion of holey optical fibre having a length Lc comprising a core region and a cladding region, said cladding region comprising, in turn, a plurality of holes passing through it longitudinally, said holes having a respective diameter and being spaced, two by two, according to a respective pitch, said unit being characterised in that it also comprises a temperature adjusting device for bringing and maintaining the holey optical fibre at temperature values T selectable in a predetermined interval of temperatures Tx-Ty.
In the present description and claims, the term xe2x80x9cpitchxe2x80x9d is used to indicate the centre-centre distance between two adjacent holes of a holey optical fibre.
Typically, the core region of the holey optical fibre portion is solid.
Advantageously, for compensating chromatic dispersion values Dt*Lt comprised in a predetermined interval of values, in a reconfigurable manner at a preselected wavelength xcex, the hole diameter and pitch, the length Lc and the interval of temperature Tx-Ty are selected so that at the preselected wavelength xcex, the holey optical fibre has such values of the chromatic dispersion coefficient Dc(T) in function of temperature that, as temperature T varies between value Tx and value Ty, the following relation Dt*Lt=xe2x88x92Dc(T)*Lc is substantially satisfied for all chromatic dispersion values Dt*Lt comprised in said predetermined interval of values.
This advantageously allows compensating, in a reconfigurable manner, and with a single unit, different values of chromatic dispersion Dt*Lt comprised in a predetermined interval of values.
Typically, the diameter and pitch of the holes of the holey optical fibre are selected so that the chromatic dispersion coefficient Dc(T) of the holey fibre is negative. This allows compensating the chromatic dispersion of conventional optical transmission fibresxe2x80x94having a positive chromatic dispersion coefficient Dtxe2x80x94such as, for example, SMF optical fibres produced, for example, by FOS or by CORNING Inc.; True Wave(trademark) (TW), True Wave Plus(trademark) (TW+) or True Wave RS(trademark) (TW RS) optical fibres produced by LUCENT Technology Inc.; large effective area (or LEAF) or LEAF Enhanced optical fibres produced by CORNING Inc., and FreeLight(trademark) optical fibres produced by FOS.
Advantageously, the chromatic dispersion coefficient Dc(T) of the holey optical fibre is negative as temperature T varies between value Tx and value Ty.
Typically, the cladding region of the holey optical fibre portion comprises a first ring of holes around the core region.
Such ring of holes can have any shape. Typically, it has a hexagonal shape. In general, it has a circular symmetry.
Advantageously, the ratio between the area taken by the first ring holes and the total area of the circular crown defined by said first ring of holes is more than 0.5. Preferably, said ratio is more than 0.6. More preferably, it is more than 0.7. In fact, the Applicant has found that the variation, as temperature changes, of the chromatic dispersion coefficient Dc(T) of the holey optical fibre increases as said ratio increases.
Typically, the holes of said first ring have a mean diameter d and are spaced from one another by a mean pitch xcex9.
Preferably, the d/xcex9 ratio is more than 0.7. Preferably, it is more than 0.8. More preferably, it is more than 0.9. In fact, the Applicant has found that the variation, as temperature changes, of the chromatic dispersion coefficient Dc(T) of the holey optical fibre increases as the d/xcex9 ratio increases.
Moreover, as described in the European patent application number 00830495.8, filed by the Applicant, the mean pitch xcex9 of the holes of said first ring is advantageously selected through the following relation (H)   Λ  =                    λ        n            ⁢              (                  1                      1            +                                          1                2                            ⁢                              d                Λ                                                    )              ±          0.25      ⁢              xe2x80x83            ⁢      μm      
where n is the refractive index of the material forming the holey optical fibre core region.
This makes the holey optical fibre to have a high absolute value of the chromatic dispersion coefficient Dc at wavelength xcex.
The Applicant has noted that relation H agrees with the corresponding values of xcex9 obtained solving the Maxwell equations in an interval of wavelengths comprised between 1300 and 1700 nm.
More in particular, relation   Λ  =            λ      n        ⁢          (              1                  1          +                                    1              2                        ⁢                          d              Λ                                          )      
provides an optimum approximation (with an error in the range of few nm) of the value of xcex9 corresponding to a maximum absolute value of Dc around the wavelength of 1550 nm, for a value of d/xcex9 of at least about 0.7, and for a hexagonal shape of the first ring of holes.
In an embodiment, all the holes of said first ring substantially have the same diameter. Moreover, said holes are substantially equally spaced from one another. This advantageously allows facilitating the fibre production process.
Advantageously, the cladding region also comprises at least a second ring of holes arranged around said first ring of holes.
Said rings of holes can have any shape. Typically, they have a hexagonal shape. In general, they have a circular symmetry.
In an embodiment, the holes of said first and said at least one second ring substantially have the same mean diameter d and are spaced from one another by the same mean pitch xcex9. This advantageously allows facilitating the fibre production process.
In an alternative embodiment, the holes of said first ring have a mean diameter that is different from the mean diameter of the holes of said at least one second ring.
Preferably, the holes of said first ring have a greater mean diameter than the mean diameter of the holes of said at least one second ring. In fact, the Applicant has noted that this allows increasing the effective area of the core region of the holey optical fibre.
Typically, said core region of the holey optical fibre portion is made of a silica-based vitreous material.
Typically, also said cladding region of the holey optical fibre portion is made of a silica-based vitreous material.
However, the cladding and core region do not necessarily consist of the same material.
Typically, the holes defined by the cladding region are filled with air. Alternatively, they are filled with another material having a lower refractive index than that of the material forming the core region. For example, such material is a gas. More in particular, a gas that does not chemically interact with the material of the core and cladding region.
Advantageously, wavelength xcex is comprised between about 1300 nm and 1700 nm. Preferably, it is comprised between about 1450 nm and 1600 nm. More preferably, it is comprised in the third optical fibre transmission band (that is, between about 1500 nm and 1600 nm). Even more preferably, it is comprised in the typical transmission band of an erbium-doped optical amplifier (that is, between about 1530 nm and 1600 nm).
Advantageously, to reduce its volume, the holey optical fibre is wound on itself and contained in a container.
The temperature adjusting device is thermally connected to said container.
Preferably, in order to allow a good heat conduction from the temperature adjusting device to the holey optical fibre, the container consists of a good temperature conductor material.
Advantageously, temperature Tx is less than temperature Ty. Moreover, the interval of temperatures Tx-Ty can be any interval selected in the interval of temperatures comprised between about xe2x88x9220xc2x0 C. and 80xc2x0 C.
Typically, temperature Tx is comprised between about xe2x88x9220 and 20xc2x0 C.
Typically, temperature Ty is comprised between about 20 and 80xc2x0 C.
Moreover, the difference between temperature Ty and temperature Tx typically is of at least about 30xc2x0 C.
To compensate a predetermined value of chromatic dispersion slope st and a predetermined value of chromatic dispersion Dt*Lt, at a preselected wavelength xcex and at a preselected temperature Tz comprised in the interval Tx-Ty, the hole diameter and pitch, the length Lc and the interval of temperature Tx-Ty are selected so that at the preselected wavelength xcex, the holey optical fibre has such values of the chromatic dispersion slope sc(T) and values of the chromatic dispersion coefficient Dc(T) in function of the temperature, as to satisfy the following relations at temperature Tz
Dt/st=Dc(Tz)/sc(Tz) and Dt*Lt=xe2x88x92Dc(Tz)*Lc.
Typically, the diameter and pitch of the holes of the holey optical fibre are selected so that also the chromatic dispersion slope sc of the holey fibre is negative. This allows compensating also the chromatic dispersion slope of conventional optical transmission fibresxe2x80x94having a positive chromatic dispersion slope stxe2x80x94such as, for example, the above SMF, True Wave(trademark), True Wave Plus(trademark), True Wave RS(trademark), LEAF, LEAF Enhanced and FreeLight(trademark) optical fibres.
Advantageously, the chromatic dispersion slope sc(T) of the holey optical fibre is negative as temperature T varies between value Tx and value Ty.
Typically, said ring of holes around the core region of the holey optical fibre substantially defines, around the core region, a circular crown having an inside radius r1 and an outside radius r2 whose values are selected so as to substantially satisfy the following relation (L)
r1xe2x89xa6xcex/nxe2x89xa6r2
where n is the refractive index of the material forming the holey optical fibre core region.
As described in the European patent application number 00830495.8, the Applicant has, in fact, found that a holey optical fibre, in which the first ring of holes defines a circular crown having such radiuses r1 and r2 that wavelength xcex/n is comprised between values r1 and r2, exhibits a negative chromatic dispersion slope sc.
Advantageously, the mean pitch value xcex9 substantially satisfies the following relation (C)             λ      n        *          1              1        +                              1            2                    ⁢                      (                          d              Λ                        )                                ≤  Λ  ≤            λ      n        *          1              1        -                              1            2                    ⁢                      (                          d              Λ                        )                              
In fact, the Applicant has found that the above relation (L) is satisfied when the mean pitch xcex9 substantially satisfies the following relation (C).
In a preferred embodiment, the unit also comprises a second portion of holey optical fibre having a predetermined length Lcxe2x80x2 comprising a core region and a cladding region, said cladding region comprising, in turn, a plurality of holes passing through it longitudinally, said holes having a respective diameter and being spaced, two by two, according to a respective pitch.
In this case, the temperature adjusting device is also advantageously suitable for bringing and maintaining the second holey optical fibre at temperature values Txe2x80x2 selectable in a second, predetermined interval of temperatures Txxe2x80x2-Tyxe2x80x2.
The Applicant has found that the unit according to this embodiment, having two holey optical fibres, has the necessary flexibility to be engineered for compensating not only chromatic dispersion values Dt*Lt comprised in a predetermined interval of values, but also a predetermined ratio Dt/st, as temperatures T and Txe2x80x2 vary, in a predetermined manner, in the intervals of temperatures Tx-Ty and Txxe2x80x2-Tyxe2x80x2.
Typically, the core region of the second holey optical fibre portion is solid.
Advantageously, for compensating chromatic dispersion values Dt*Lt comprised in a predetermined interval of values, in a reconfigurable manner at the preselected wavelength xcex, the hole diameter and pitch, the lengths Lc and Lcxe2x80x2, and the intervals of temperature Tx-Ty and Txxe2x80x2-Tyxe2x80x2 are advantageously selected so that at the preselected wavelength xcex, the first and the second holey optical fibre have such values of the chromatic dispersion coefficient Dc(T) and, respectively, Dc(Txe2x80x2), in function of temperature that, as temperatures T, Txe2x80x2 vary, in a predetermined manner, between value Tx and value Ty, and respectively, between value Txxe2x80x2 and Tyxe2x80x2, the following relation (K) is substantially satisfied
Dt*Lt=xe2x88x92[Dc(T)*Lc+Dc(Txe2x80x2)*Lcxe2x80x2]
for all values of chromatic dispersion Dt*Lt comprised in said predetermined interval of values.
To compensate also a predetermined ratio Dt/st between the chromatic dispersion coefficient Dt and a chromatic dispersion slope st, at the preselected wavelength xcex and as temperatures T, Txe2x80x2 vary, the hole diameter and pitch of the first and second holey fibre, the lengths Lc and Lcxe2x80x2 and the intervals of temperature Tx-Ty and Txxe2x80x2-Tyxe2x80x2 are also advantageously selected so that, at the preselected wavelength xcex, the first and the second holey optical fibres have such values of the chromatic dispersion slope sc(T) and, respectively, sc(Txe2x80x2) and values of the chromatic dispersion coefficient Dc(T) and, respectively, Dc(Txe2x80x2) in function of temperatures T and Txe2x80x2 that, as temperatures T, Txe2x80x2 vary in said predetermined manner between value Tx and value Ty and, respectively, between value Txxe2x80x2 and value Tyxe2x80x2, also the following relation (J) is substantially satisfied.             D      ⁢              xe2x80x83            ⁢      t              s      ⁢              xe2x80x83            ⁢      t        =                                                        D              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                  T                  )                                                                    s              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                  T                  )                                                              *          L          ⁢                      xe2x80x83                    ⁢          c                +                                            D              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                                      T                    xe2x80x2                                    )                                                                    s              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                                      T                    xe2x80x2                                    )                                                              *          L          ⁢                      xe2x80x83                    ⁢                      c            xe2x80x2                                                L          ⁢                      xe2x80x83                    ⁢          c                +                  L          ⁢                      xe2x80x83                    ⁢                      c            xe2x80x2                                .  
This last embodiment of the invention advantageously allows compensating not only chromatic dispersion values Dt*Lt comprised in a predetermined interval of values, but also a predetermined ratio Dt/st, as temperatures T and Txe2x80x2 vary, in a predetermined manner, in the intervals of temperatures Tx-Ty and Txxe2x80x2-Tyxe2x80x2.
In other words, this last embodiment allows compensating both the chromatic dispersion slope and the chromatic dispersion coefficient of optical transmission fibres having the same values of chromatic dispersion slope st and of chromatic dispersion coefficient Dt but different lengths Lt (that is, having equal values of the Dt/st ratio but different chromatic dispersion values Dt*Lt).
For compensating also values of the Dt/st ratio comprised in a preselected interval of values, at the preselected wavelength xcex and as temperatures T, Txe2x80x2 vary, besides chromatic dispersion values Dt*Lt comprised in a predetermined interval of values, the hole diameter and pitch of the first and second holey fibre, the lengths Lc and Lcxe2x80x2 and the intervals of temperature Tx-Ty and Txxe2x80x2-Tyxe2x80x2 are also advantageously selected so that, at the preselected wavelength xcex, the first and the second holey optical fibre have such values of the chromatic dispersion slope sc(T) and, respectively, sc(Txe2x80x2) and values of the chromatic dispersion coefficient Dc(T) and, respectively, Dc(Txe2x80x2) in function of temperatures T and Txe2x80x2 that, as temperatures T, Txe2x80x2 vary in a predetermined manner between value Tx and value Ty and, respectively, between value Txxe2x80x2 and value Tyxe2x80x2, also the following relation (J) is substantially satisfied             D      ⁢              xe2x80x83            ⁢      t              s      ⁢              xe2x80x83            ⁢      t        =                                          D            ⁢                          xe2x80x83                        ⁢                          c              ⁢                              (                T                )                                                          s            ⁢                          xe2x80x83                        ⁢                          c              ⁢                              (                T                )                                                    *        L        ⁢                  xe2x80x83                ⁢        c            +                                    D            ⁢                          xe2x80x83                        ⁢                          c              ⁢                              (                                  T                  xe2x80x2                                )                                                          s            ⁢                          xe2x80x83                        ⁢                          c              ⁢                              (                                  T                  xe2x80x2                                )                                                    *        L        ⁢                  xe2x80x83                ⁢                  c          xe2x80x2                                    L        ⁢                  xe2x80x83                ⁢        c            +              L        ⁢                  xe2x80x83                ⁢                  c          xe2x80x2                    
for all values of the Dt/st ratio comprised in said preselected interval of values.
This last embodiment of the invention advantageously allows compensating not only chromatic dispersion values Dt*Lt comprised in a predetermined interval of values, but values of the Dt/st ratio comprised in a preselected interval of values, as temperatures T and Txe2x80x2 vary, in a predetermined manner, in the intervals of temperatures Tx-Ty and Txxe2x80x2-Tyxe2x80x2.
In other words, this last embodiment allows compensating both the chromatic dispersion slope and the chromatic dispersion coefficient of an optical transmission fibre having a fixed length Lt but variable values of chromatic dispersion slope st and of chromatic dispersion coefficient Dt with respect to the nominal value (due, for example, to variations of system parameters, such as temperature).
Typically, the cladding region of the second holey optical fibre defines a first ring of holes around the core region. Moreover, the holes of said first ring have a mean diameter dxe2x80x2 and are spaced from one another by a mean pitch xcex9xe2x80x2.
Moreover, the d/xcex9 ratio of the first holey optical fibre and the dxe2x80x2/xcex9xe2x80x2 ratio of the second holey optical fibre are advantageously different from each another.
In this way, the chromatic dispersion coefficients Dc(T) and Dc(Txe2x80x2) of the two portions of holey optical fibre vary in a different way as the temperature varies.
Advantageously, ratio d/xcex9 of the first holey optical fibre is of at least 0.7. Preferably, it is of at least 0.8. More preferably, it is of at least 0.9.
Advantageously, ratio d/xcex9xe2x80x2 of the second holey optical fibre is comprised between 0.4 and 0.7. Preferably, it is comprised between 0.4 and 0.6.
Moreover, in the unit with two portions of optical fibre, the temperature adjusting device can consist of two temperature adjusting apparatuses, one for bringing and maintaining the first holey optical fibre at temperature values selectable in the first predetermined interval of temperatures Tx-Ty and the other for bringing and maintaining the second holey optical fibre at temperature values selectable in the second predetermined interval of temperatures Txxe2x80x2-Tyxe2x80x2.
In a second aspect thereof, the present invention further relates to an optical communication line comprising
a portion of optical transmission fibre having a length Lt and a chromatic dispersion coefficient Dt at a wavelength xcex; and
a unit comprising, in turn, a portion of holey optical fibre having a predetermined length Lc and comprising a core region and a cladding region, said cladding region comprising, in turn, a plurality of holes passing through it longitudinally, said holes having a respective diameter and being spaced, two by two, according to a respective pitch,
characterised in that
said unit also comprises a temperature adjusting device for bringing and maintaining the holey optical fibre at a temperature value Tz selectable in a predetermined interval of temperatures Tx-Ty; and in that
the hole diameter and pitch, the length Lc, the temperature value Tz and the interval of temperatures Tx-Ty are selected so that the holey optical fibre has such value of the chromatic dispersion coefficient Dc(Tz) at the preselected wavelength xcex and at the temperature value Tz, as to compensate the chromatic dispersion of the optical transmission fibre portion.
Typically, the hole diameter and pitch, the length Lc, the temperature value Tz and the interval of temperatures Tx-Ty are selected so that the chromatic dispersion coefficient Dc(Tz) has such value as to substantially satisfy the following relation Dt*Lt=xe2x88x92Dc(Tz)*Lc.
Typically, in said optical communication line
the portion of optical transmission fibre also has a chromatic dispersion slope st at wavelength xcex, and
in the unit, the hole diameter and pitch, the length Lc, the temperature value Tz and the interval of temperature Tx-Ty are also selected so that the holey optical fibre has such value of the chromatic dispersion slope sc(Tz) and a value of the chromatic dispersion coefficient Dc(Tz) at the preselected wavelength xcex and at the temperature value Tz, that also the following relation Dt/st=Dc(Tz)/sc(Tz) is substantially satisfied, so as to substantially compensate also the chromatic dispersion slope of the optical transmission fibre portion.
In the event that the portion of optical transmission fibre has a variable chromatic dispersion Dt*Lt in a predetermined interval of values, the hole diameter and pitch, the length Lc and the interval of temperatures Tx-Ty are also advantageously selected so that at the preselected wavelength xcex, the holey optical fibre has such values of the chromatic dispersion coefficient Dc(T) in function of temperature that, as temperature T varies between value Tx and value Ty, the following relation Dt*Lt=xe2x88x92Dc(T)*Lc is substantially satisfied for all chromatic dispersion values Dt*Lt comprised in said predetermined interval of values.
Preferably, the unit also comprises a second portion of holey optical fibre having a predetermined length Lcxe2x80x2 comprising a core region and a cladding region, said cladding region comprising, in turn, a plurality of holes passing through it longitudinally, said holes having a respective diameter and being spaced, two by two, according to a respective pitch.
In this case, the temperature adjusting device is also advantageously suitable for bringing and maintaining the second holey optical fibre at a temperature value Tzxe2x80x2 selectable in a second, predetermined interval of temperatures Txxe2x80x2-Tyxe2x80x2.
For compensating the chromatic dispersion Dt*Lt of the optical transmission fibre portion, the hole diameter and pitch of the first and second holey fibre, the lengths Lc and Lcxe2x80x2, the values of temperature Tz and Tzxe2x80x2 and the intervals of temperature Tx-Ty and Txxe2x80x2-Tyxe2x80x2 are advantageously selected so that at the preselected wavelength xcex, the first and the second holey optical fibre have such values of the chromatic dispersion coefficient Dc(Tz) and, respectively, Dc(Tzxe2x80x2) that the following relation is substantially satisfied
Dt*Lt=xe2x88x92[Dc(Tz)*Lc+Dc(Tzxe2x80x2)*Lcxe2x80x2].
For compensating also the chromatic dispersion slope st of the optical transmission fibre portion, the hole diameter and pitch of the first and second holey fibre, the lengths Lc and Lcxe2x80x2, the values of temperature Tz and Tzxe2x80x2 and the intervals of temperature Tx-Ty and Txxe2x80x2-Tyxe2x80x2 are also advantageously selected so that at the preselected wavelength xcex, the first and the second holey optical fibre have such values of the chromatic dispersion slope sc(Tz) and, respectively, sc(Tzxe2x80x2), and values of the chromatic dispersion coefficient Dc(Tz) and, respectively, Dc(Tzxe2x80x2), that also the following relation is substantially satisfied.             D      ⁢              xe2x80x83            ⁢      t              s      ⁢              xe2x80x83            ⁢      t        =                                                        D              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                                      T                    ⁢                                          xe2x80x83                                        ⁢                    z                                    )                                                                    s              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                                      T                    ⁢                                          xe2x80x83                                        ⁢                    z                                    )                                                              *          L          ⁢                      xe2x80x83                    ⁢          c                +                                            D              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                                      T                    ⁢                                          xe2x80x83                                        ⁢                                          z                      xe2x80x2                                                        )                                                                    s              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                                      T                    ⁢                                          xe2x80x83                                        ⁢                                          z                      xe2x80x2                                                        )                                                              *          L          ⁢                      xe2x80x83                    ⁢                      c            xe2x80x2                                                L          ⁢                      xe2x80x83                    ⁢          c                +                  L          ⁢                      xe2x80x83                    ⁢                      c            xe2x80x2                                .  
In the event that the optical transmission fibre portion has a variable chromatic dispersion Dt*Lt in a predetermined interval of values, the hole diameter and pitch of the first and second holey fibre, the lengths Lc and Lcxe2x80x2, and the intervals of temperature Tx-Ty and Txxe2x80x2-Tyxe2x80x2 are advantageously selected so that at the preselected wavelength xcex, the first and the second holey optical fibre have such values of the chromatic dispersion coefficient Dc(T) and, respectively, Dc(Txe2x80x2), as temperatures T, Txe2x80x2 vary, in a predetermined manner, between values Tx and Ty, and respectively, Txxe2x80x2 and Tyxe2x80x2, that the following relation (K) is substantially satisfied
Dt*Lt=xe2x88x92[Dc(T)*Lc+Dc(Txe2x80x2)*Lcxe2x80x2]
for all values of chromatic dispersion Dt*Lt comprised in the predetermined interval of values.
Moreover, if the optical transmission fibre portion also has a variable Dt/st ratio in a preselected interval of values, the hole diameter and pitch of the first and second holey fibre, the lengths Lc and Lcxe2x80x2 and the intervals of temperature Tx-Ty and Txxe2x80x2-Tyxe2x80x2 are also selected so that, at the preselected wavelength xcex, the first and the second holey optical fibres have such values of the chromatic dispersion slope sc(T) and, respectively, sc(Txe2x80x2) and values of the chromatic dispersion coefficient Dc(T) and, respectively, Dc(Txe2x80x2) that, as temperatures T, Txe2x80x2 vary in said predetermined manner between value Tx and value Ty and, respectively, between value Txxe2x80x2 and value Tyxe2x80x2, also the following relation (J) is substantially satisfied             D      ⁢              xe2x80x83            ⁢      t              s      ⁢              xe2x80x83            ⁢      t        =                                          D            ⁢                          xe2x80x83                        ⁢                          c              ⁢                              (                T                )                                                          s            ⁢                          xe2x80x83                        ⁢                          c              ⁢                              (                T                )                                                    *        L        ⁢                  xe2x80x83                ⁢        c            +                                    D            ⁢                          xe2x80x83                        ⁢                          c              ⁢                              (                                  T                  xe2x80x2                                )                                                          s            ⁢                          xe2x80x83                        ⁢                          c              ⁢                              (                                  T                  xe2x80x2                                )                                                    *        L        ⁢                  xe2x80x83                ⁢                  c          xe2x80x2                                    L        ⁢                  xe2x80x83                ⁢        c            +              L        ⁢                  xe2x80x83                ⁢                  c          xe2x80x2                    
for all values of the Dt/st ratio comprised in said preselected interval of values.
In general, as regards the features of the unit, of the first and second holey optical fibre portion, of the holes of cladding regions, and of the temperature adjusting device, reference shall be made to what already described with reference to the unit according to the first aspect of the invention.
The portion of holey optical fibre can be arranged inside, upstream or downstream of said optical transmission fibre portion. Advantageously, it is arranged downstream of said optical transmission fibre portion. This allows limiting the non-linear effects of the holey optical fibre since, in this case, an optical signal in input to such fibre has a relatively low optical power due to the attenuation undergone along the upstream optical transmission fibre portion.
Typically, said optical communication line also comprises an optical amplifier.
Preferably, said optical amplifier is arranged downstream of said holey optical fibre portion. This allows amplifying the optical signal after passing along the holey optical fibre, so that, when in input to the holey optical fibre, it has a relatively low optical power due to the attenuation undergone along the optical transmission fibre portion. In this way, non-linear effects on the signal by the holey optical fibre are limited.
Advantageously, said amplifier is of the active optical fibre type, doped with a rare earth.
Typically, said rare earth is erbium.
Typically, said portion of optical transmission line is a conventional optical fibre essentially consisting of a core and a cladding having a lower refractive index than that of the core, so as to confine the transmitted signal into the latter.
Typically, both the core and the cladding are made of a silica-based vitreous material, and the difference of refractive index between core and cladding is obtained by incorporating suitable dopants (such as, for example, germanium, phosphorus and/or fluorine) in the vitreous matrix of the core and/or cladding.
Typically, said optical transmission fibre portion is a conventional single mode optical fibre (or SMF) produced, for example, by FOS or by CORNING Inc..
Advantageously, said optical transmission fibre portion is selected from the group comprising a True Wave(trademark) (TW) optical fibre, a True Wave Plus(trademark) (TW+) optical fibre, a True Wave RS(trademark) (TW RS) optical fibre produced by LUCENT Technology Inc.; a large effective area optical fibre (or LEAF), a LEAF Enhanced optical fibre produced by CORNING Inc., and a FreeLight(trademark) optical fibre produced by FOS.
In a third aspect thereof, the present invention further relates to an optical communication system comprising
a transmitting suitable for providing an optical signal having a signal wavelength xcex;
an optical communication line, optically connected to said transmitting station, for transmitting said optical signal, said line comprising at least one portion of optical transmission fibre, having a length Lt and a chromatic dispersion coefficient Dt at wavelength xcex, and a unit comprising, in turn, a portion of holey optical fibre having a predetermined length Lc and comprising a core region and a cladding region, said cladding region comprising, in turn, a plurality of holes passing through it longitudinally, said holes having a respective diameter and being spaced, two by two, according to a respective pitch,
a receiving station, optically connected to said optical communication line, for receiving said optical signal,
characterised in that
said unit also comprises a temperature adjusting device for bringing and maintaining the holey optical fibre at a temperature value Tz selectable in a predetermined interval of temperatures Tx-Ty; and in that
the hole diameter and pitch, the length Lc, the temperature value Tz and the interval of temperatures Tx-Ty are selected so that the holey optical fibre has such value of the chromatic dispersion coefficient Dc(Tz) at the preselected wavelength xcex and at the temperature value Tz, as to compensate the chromatic dispersion of the optical transmission fibre portion.
In general, as regards the features of the optical communication line and of the unit, reference shall be made to what already described with reference to the first and second aspect of the invention.
Typically, said optical signal is a WDM optical signal comprising a plurality of N signals having wavelengths xcex1, xcex2 . . . xcexN.
In a further aspect thereof, the present invention also relates to a use, for compensating the chromatic dispersion in a reconfigurable manner, of a unit comprising a portion of holey optical fibre having a length Lc comprising a core region and a cladding region, said cladding region comprising, in turn, a plurality of holes passing through it longitudinally, said holes having a respective diameter and being spaced, two by two, according to a respective pitch, said unit being characterised in that it also comprises a temperature adjusting device for bringing and maintaining the holey optical fibre at temperature values T selectable in a predetermined interval of temperatures Tx-Ty so that the holey optical fibre takes desired chromatic dispersion values Dc(T).
As regards the features of the unit, reference shall be made to what described above with reference to the first aspect of the invention.
In a further aspect thereof, the present invention also relates to a method for compensating chromatic dispersion in a reconfigurable manner, comprising the steps of
a) providing a holey optical fibre having a length Lc and comprising a core region and a cladding region, said cladding region comprising, in turn, a plurality of holes passing through it longitudinally, said holes having a respective diameter and being spaced, two by two, according to a respective pitch;
b) bringing and maintaining said holey optical fibre at a temperature T selectable in a predetermined interval of temperatures Tx-Ty so that the holey optical fibre has a desired chromatic dispersion value Dc(T).
Advantageously, for compensating chromatic dispersion values Dt*Lt comprised in a predetermined interval of values, in a reconfigurable manner at a preselected wavelength xcex, the method also comprises the step c) of selecting the hole diameter and pitch, the length Lc and the interval of temperature Tx-Ty so that at the preselected wavelength xcex, the holey optical fibre has such values of the chromatic dispersion coefficient Dc(T) in function of temperature that, as temperature T varies between value Tx and value Ty, the following relation Dt*Lt=xe2x88x92Dc(T)*Lc is substantially satisfied for all chromatic dispersion values Dt*Lt comprised in said predetermined interval of values.
To compensate both a predetermined value of chromatic dispersion slope st and a predetermined value of chromatic dispersion Dt, at a preselected wavelength xcex and at a preselected temperature Tz comprised in the interval Tx-Ty, the method of the invention also comprises the step d) of selecting the hole diameter and pitch, the length Lc and the interval of temperature Tx-Ty so that at the preselected wavelength xcex, the holey optical fibre has such value of the chromatic dispersion slope sc and such value of the chromatic dispersion coefficient Dc in function of the temperature, as to substantially satisfy the following relation Dt/st=Dc(Tz)/sc(Tz).
In a preferred embodiment, the method of the invention also comprises the step e) of providing also a second portion of holey optical fibre having a predetermined length Lcxe2x80x2 comprising a core region and a cladding region, said cladding region comprising, in turn, a plurality of holes passing through it longitudinally, said holes having a respective diameter and being spaced, two by two, according to a respective pitch.
In this case, the method of the invention also comprises the step f) of bringing and maintaining the second holey optical fibre at a temperature value Txe2x80x2 selectable in a second, predetermined interval of temperatures Txxe2x80x2-Tyxe2x80x2.
For compensating chromatic dispersion values Dt*Lt comprised in a predetermined interval of values, at the preselected wavelength xcex, in place of step c) the method of the invention comprises the step g) of selecting the diameter and pitch of the holes of the first and second holey fibre, the lengths Lc and Lcxe2x80x2, and the intervals of temperature Tx-Ty and Txxe2x80x2-Tyxe2x80x2 so that at the preselected wavelength xcex, the first and the second holey optical fibre have such values of the chromatic dispersion coefficient Dc(T) and, respectively, Dc(Txe2x80x2), in function of temperature that, as temperatures T, Txe2x80x2 vary, in a predetermined manner, between value Tx and value Ty, and respectively, between value Txxe2x80x2 and Tyxe2x80x2, the following relation (K) is substantially satisfied
Dt*Lt=xe2x88x92[Dc(T)*Lc+Dc(Txe2x80x2)*Lcxe2x80x2],
for all values of chromatic dispersion Dt*Lt comprised in said predetermined interval of values.
To compensate also a predetermined ratio Dt/st at the preselected wavelength xcex and as temperatures T, Txe2x80x2 vary, in place of step d) the method of the invention also comprises the step h) of selecting the hole diameter and pitch of the first and second holey fibre, the lengths Lc and Lcxe2x80x2 and the intervals of temperature Tx-Ty and Txxe2x80x2-Tyxe2x80x2 so that, at the preselected wavelength xcex, the first and the second holey optical fibres have such values of the chromatic dispersion slope sc(T) and, respectively, sc(Txe2x80x2) and values of the chromatic dispersion coefficient Dc(T) and, respectively, Dc(Txe2x80x2) in function of temperatures T and Txe2x80x2 that, as temperatures T, Txe2x80x2 vary in said predetermined manner between value Tx and value Ty and, respectively, between value Txxe2x80x2 and value Tyxe2x80x2, also the following relation (J) is substantially satisfied.             D      ⁢              xe2x80x83            ⁢      t              s      ⁢              xe2x80x83            ⁢      t        =                                                        D              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                  T                  )                                                                    s              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                  T                  )                                                              *          L          ⁢                      xe2x80x83                    ⁢          c                +                                            D              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                                      T                    xe2x80x2                                    )                                                                    s              ⁢                              xe2x80x83                            ⁢                              c                ⁢                                  (                                      T                    xe2x80x2                                    )                                                              *          L          ⁢                      xe2x80x83                    ⁢                      c            xe2x80x2                                                L          ⁢                      xe2x80x83                    ⁢          c                +                  L          ⁢                      xe2x80x83                    ⁢                      c            xe2x80x2                                .  